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Abstract:

Disclosed is a printed wiring board offering improved reliability through
increased mechanical strength at the bottom of cavity areas for mounting
components. A printed wiring board 10 is characterized in that an
insulation layer 16 is formed on either the top or bottom side of a metal
core 11, while an opening 12 formed in the metal core 11 is used as a
cavity area 15a for mounting a component, wherein a reinforcement pattern
30 is formed on the surface of an insulation layer facing the bottom of
the cavity area 15a in the insulation layer 16. The reinforcement pattern
30 is made of the same material as the wiring patterns 28c, 29c formed on
the insulation layer 16, and also formed simultaneously with these wiring
patterns 28c, 29c.

Claims:

1. A printed wiring board with a cavity area for mounting an electronic
component, where the cavity area is formed by providing, in a
sheet-shaped metal core, a through-opening connecting its two principle
sides in a location where an electronic component is to be stored, and
then forming an insulation layer on one principle side of the metal core
to cover one end of the opening; wherein on one side of the insulation
layer not contacting the metal core, wiring patterns for connecting
electrodes of the electronic component are provided, and a reinforcement
pattern not contacting the wiring patterns is formed in an approximate
area corresponding to the cavity.

2. A printed wiring board according to claim 1, wherein the reinforcement
pattern is made of the same material as the wiring patterns formed on the
same side, and also formed simultaneously with these wiring patterns.

3. A printed wiring board according to claim 1, wherein the reinforcement
pattern is formed wider than the area of the opening.

4. A printed wiring board according to claim 1, wherein the reinforcement
pattern is a rectangle in a plan view.

5. A printed wiring board according to claim 4, wherein in addition to
the insulation layer as a first insulation layer, a second insulation
layer is formed on the first insulation layer, the wiring patterns, and
the reinforcement pattern, and the reinforcement pattern has many small
holes through which the first and second insulation layers are connected
so as to increase adhesion of the first and second insulation layers.

6. A printed wiring board according to claim 1, wherein the reinforcement
pattern is divided in a plan view.

7. A method for manufacturing a printed wiring board, comprising: a step
to make a ring-shaped opening in a sheet-shaped metal core, which opening
is interrupted at a bridge part, to form an island-like shape; a step to
form, on one principle side of the metal core, a wiring layer having an
insulation layer and wiring patterns; a step to form, on the other
principle side of the metal core, a wiring layer having an insulation
layer, wiring patterns, and a reinforcement pattern; a step to open an
area corresponding to the island-like shape in one of the wiring layers;
and a step to remove the island-like shape by utilizing this opening in
the area corresponding to the island-like shape.

8. A method for manufacturing a printed wiring board according to claim
7, further comprising a step to fill with an insulator the opening around
the island-like shape when the insulation layer is formed on the wiring
layer on the one or the other principle side.

9. A method for manufacturing a printed wiring board according to claim
7, wherein the reinforcement pattern is made of the same material as the
wiring patterns formed on the same side, and also formed simultaneously
with these wiring patterns.

10. A method for manufacturing a printed wiring board according to claim
7, wherein the reinforcement pattern is formed wider than the area of the
opening corresponding to the island-like shape.

11. A method for manufacturing a printed wiring board, comprising: a step
to form, on one principle side of a sheet-shaped metal core, a wiring
layer having an insulation layer and wiring patterns formed on this
insulation layer; a step to form, on the other principle side of the
metal core, a wiring layer having an insulation layer, a wiring pattern
formed on this insulation layer, and a reinforcement pattern; a step to
open an area where an electronic component is to be stored, in the wiring
layer formed on the one principle side; and a step to utilize this
opening to form in the metal core an opening roughly as wide as the
opening.

12. A method for manufacturing a printed wiring board according to claim
11, wherein the step to form an opening in the metal core uses etching to
form the opening.

13. A method for manufacturing a printed wiring board according to claim
11, wherein the reinforcement pattern is made of the same material as the
wiring patterns formed on the same side, and also formed simultaneously
with these wiring patterns.

14. A method for manufacturing a printed wiring board according to claim
11, wherein the reinforcement pattern is formed wider than the area of
the opening corresponding to the area where the electronic component is
to be stored.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a wiring board, and more
specifically to a printed wiring board having cavity storage areas for
mounting electronic components, as well as a method for manufacturing the
same.

BACKGROUND ART

[0002] Demand for thinner electronic devices is growing in recent years,
and there is a need for making internal components, especially printed
wiring boards on which electronic components are mounted, even thinner.
Since the thickness of a printed wiring board on which electronic
components are mounted (hereinafter referred to as "A" for the sake of
convenience) is given as a sum of the thickness of the printed wiring
board itself (hereinafter referred to as "B" for the sake of convenience)
and height of the components (hereinafter referred to as "C" for the sake
of convenience) (A=B+C), the aforementioned need can be met by reducing B
or C or both. However, how much B (thickness of printed wiring board) and
C (height of components) can be reduced is limited, and the industry has
been waiting for breakthrough measures.

[0003] With regard to this point, Patent Literature 1 (hereinafter
referred to as the "background art") listed below describes a technology
to form cavity areas on a printed wiring board and mount components in
these cavity areas. If the depth of the cavity area is D, for example,
this background art provides the same effect as reducing the height of
components C by D, and effectively reduces, to a substantial degree, the
thickness of component-mounted printed wiring board A.

[0004] With a printed wiring board conforming to the aforementioned
background art, however, the "bottom" of cavity areas for mounting
electronic components is not very strong and, when electronic components
are mounted in the cavity areas, these cavity areas for mounting
components will crack due to the pressure applied to the component
surface if a strong, flat jig is not placed below the insulation layer
forming the bottom, or if the height of components C exceeds the depth of
cavity area D. In the worst case scenario, the bottom may come off. This
problem can also occur when the height of components C is less than the
depth of cavity area D.

[0005] FIG. 18 shows the structure of the background art. In this figure,
a printed wiring board 1 is constituted by a metal sheet 2 having, on one
side of it, a resin film 3 and insulation sheet 4 attached on top of each
other, as well as an electronic component 6 mounted in a cavity area 5
formed on this metal sheet 2. Here, A is the thickness of the printed
wiring board 1 on which the component is mounted, B is the thickness of
the printed wiring board 1 itself, C is the height of the component 6,
and D is the depth of the cavity area 5. Here, the magnitude correlation
"C>D" holds true, meaning that a part of the component 6 is projecting
from the cavity area 5.

[0006] In this condition, with the electronic component mounted, an
unwanted pressure P may be applied to the surface of the component 6 when
the printed wiring board 1 is assembled into an electronic device. A
similar pressure P may also be applied, even after the printed wiring
board 1 has been assembled, to the surface of the component 6 via an
enclosure of the electronic device.

[0007] The mechanical strength of the printed wiring board 1 is primarily
assured by the metal sheet 2, but the strength of the location where this
metal sheet 2 is missing, or specifically a bottom 5a of the cavity area
5, depends on the strength of the resin film 3 and insulation sheet 4
that are much more fragile than the metal sheet 2, and consequently the
bottom 5a of this cavity area 5 may detach depending on the degree of the
aforementioned pressure P.

[0008] As for printed wiring boards having cavity areas for mounting
components, there is a need in the market, of late, for ultra-thin boards
that were not before required, such as boards of 1 mm or less in
thickness. With these ultra-thin printed wiring boards, the
aforementioned problem of mechanical fragility becomes more serious. If
the insulation layer at the bottom of the cavity area is only several
tenths of a millimeter thick, the bottom may crack or detach even with a
very small force.

[0009] When printed wiring boards having cavity areas for mounting
components began being available on the market, these boards were much
thicker than 1 mm. Accordingly, forming the cavity area by machining the
board, for example, was fairly easy. On the other hand, to form a cavity
area on an ultra-thin printed board whose thickness is only 1 mm or even
less, first and foremost it is necessary to overcome the aforementioned
problem (mechanical fragility at the bottom of the cavity area), because
unless this problem is overcome, the above market need of late cannot be
met.

[0010] For example, current technology is sufficient to form a cavity area
of 0.4 mm in depth on a board of 0.5 mm in thickness. A module can be
made thinner by the depth of this cavity area. This thickness reduction
of only 0.4 mm or so is enough to meet the market need of late. Rather, a
primary reason why this market need cannot be met is the aforementioned
mechanical fragility at the bottom of the concaved area.

[0012] In light of the above, the object of the present invention is to
provide a printed wiring board offering improved reliability through
increased mechanical strength at the bottom of cavity areas for mounting
components, as well as a method for manufacturing such printed wiring
board.

MEANS FOR SOLVING THE PROBLEMS

[0013] To achieve the aforementioned object, the invention from a first
aspect is a printed wiring board with a cavity area for mounting an
electronic component, where the cavity area is formed by providing, in a
sheet-shaped metal core, a through-opening connecting its two principle
sides in a location where an electronic component is to be stored, and
then forming an insulation layer on one principle side of the metal core
to cover one end of the opening; wherein, on one side of the insulation
layer not contacting the metal core, wiring patterns for connecting
electrodes of the electronic component are provided, and a reinforcement
pattern not contacting the wiring patterns is formed in an approximate
area corresponding to the cavity.

[0014] The invention from a second aspect is a printed wiring board
according to the invention from the first aspect, wherein the
reinforcement pattern is made of the same material as the wiring patterns
formed on the same side, and also formed simultaneously with these wiring
patterns.

[0015] The invention from a third aspect is a printed wiring board
according to the invention from the first aspect, wherein the
reinforcement pattern is formed wider than the area of the opening.

[0016] The invention from a fourth aspect is a printed wiring board
according to the invention from the first aspect, wherein the
reinforcement pattern is a rectangle in a plan view.

[0017] The invention from a fifth aspect is a printed wiring board
according to the invention from the fourth aspect, wherein the
reinforcement pattern has many small holes.

[0018] The invention from a sixth aspect is a printed wiring board
according to the invention from the first aspect, wherein the
reinforcement pattern is divided in a plan view.

[0019] The invention from a seventh aspect is a method for manufacturing
printed wiring board comprising: [0020] a step to make a ring-shaped
opening in a sheet-shaped metal core, which opening is interrupted at a
bridge part, to form an island-like shape; [0021] a step to form, on one
principle side of the metal core, a wiring layer having an insulation
layer and wiring patterns; [0022] a step to form, on the other principle
side of the metal core, a wiring layer having an insulation layer, wiring
patterns and a reinforcement pattern; [0023] a step to open an area
corresponding to the island-like shape in one of the wiring layers; and
[0024] a step to remove the island-like shape by utilizing this opening.

[0025] The invention from a eighth aspect is a method for manufacturing
printed wiring board according to the invention from the seventh aspect,
further comprising a step to fill with an insulator the opening around
the island-like shape when the insulation layer in the wiring layer is
formed.

[0026] The invention from a ninth aspect is a method for manufacturing
printed wiring board according to the invention from the seventh aspect,
wherein the reinforcement pattern is made of the same material as the
wiring patterns formed on the same side, and also formed simultaneously
with these wiring patterns.

[0027] The invention from a tenth aspect is a method for manufacturing
printed wiring board according to the invention from the seventh aspect,
wherein the reinforcement pattern is formed wider than the area of the
opening corresponding to the island-like shape.

[0028] The invention from an eleventh aspect is a method for manufacturing
printed wiring board comprising: [0029] a step to form, on one principle
side of a sheet-shaped metal core, a wiring layer having an insulation
layer and wiring patterns formed on this insulation layer; [0030] a step
to form, on the other principle side of the metal core, a wiring layer
having an insulation layer, wiring pattern formed on this insulation
layer, and a reinforcement pattern; [0031] a step to open an area where
an electronic component is to be stored, in the wiring layer formed on
the one principle side; and [0032] a step to utilize this opening to form
in the metal core an opening roughly as wide as the aforementioned
opening.

[0033] The invention from a twelfth aspect is a method for manufacturing
printed wiring board according to the invention from the ninth aspect,
wherein the step to form an opening in the metal core uses etching to
form the opening.

[0034] The invention from a thirteenth aspect is a method for
manufacturing printed wiring board according to the invention from the
eleventh aspect, wherein the reinforcement pattern is made of the same
material as the wiring patterns formed on the same side, and also formed
simultaneously with these wiring patterns.

[0035] The invention from a fourteenth aspect is a method for
manufacturing printed wiring board according to the invention from the
eleventh aspect, wherein the reinforcement pattern is formed wider than
the area of the opening corresponding to the island-like shape.

EFFECTS OF THE INVENTION

[0036] According to the present invention, a printed wiring board offering
improved reliability through increased mechanical strength at the bottom
of cavity areas for mounting components, as well as a method for
manufacturing such printed wiring board, can be provided.

[0037] The aforementioned object and other objects,
constitutions/characteristics and operations/effects of the present
invention are revealed in greater detail by the explanation below and
attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] FIG. 1 is a section view of the printed wiring board in the first
embodiment, cut along a line crossing the centers of cavity areas for
storing electronic components.

[0039] FIG. 2 is a plan view of the reinforcement pattern 30 corresponding
to the plan over the area of view A-A in FIG. 1 as viewed from the bottom
side.

[0040] FIG. 3 illustrates a manufacturing process diagram for the printed
wiring board in the first embodiment (first to third steps).

[0041] FIG. 4 illustrates a manufacturing process diagram for the printed
wiring board in the first embodiment (fourth to sixth steps).

[0042] FIG. 5 illustrates a manufacturing process diagram for the printed
wiring board in the first embodiment (seventh to ninth steps).

[0043] FIG. 6 illustrates a manufacturing process diagram for the printed
wiring board in the first embodiment (tenth to twelfth steps).

[0044] FIG. 7 illustrates a manufacturing process diagram for the printed
wiring board in the first embodiment (thirteenth to fifteenth steps).

[0045] FIG. 8 is a partial plan view showing the island-like shape 38 in
the metal core of the printed wiring board in the first embodiment.

[0046] FIG. 9 illustrates a manufacturing process diagram for the printed
wiring board in the first embodiment, showing the sixteenth step.

[0047] FIG. 10 is a drawing showing the first variation of the
reinforcement pattern 30.

[0048] FIG. 11 is a drawing showing the second variation of the
reinforcement pattern 30.

[0049] FIG. 12 is a drawing showing the third variation (a) and fourth
variation (b) of the reinforcement pattern 30.

[0050] FIG. 13 is a drawing showing the fifth variation of the
reinforcement pattern 30, where (a) is a plan view of the variation and
(b) is a B-B section view of (a).

[0051] FIG. 14 is a drawing illustrating the sixth variation of the
reinforcement pattern 30.

[0052] FIG. 15 illustrates a manufacturing process diagram for the printed
wiring board in the second embodiment (first to third steps).

[0053] FIG. 16 illustrates a manufacturing process diagram for the printed
wiring board in the second embodiment (fourth to sixth steps).

[0054] FIG. 17 illustrates a manufacturing process diagram for the printed
wiring board in the second embodiment (seventh to ninth steps).

[0055] FIG. 18 is a schematic structural diagram of the background art.

MODE FOR CARRYING OUT THE INVENTION

[First Embodiment]

[0056] Embodiments of the present invention are explained below by
referring to the drawings.

[0057] First, the structure is explained.

[0058] FIG. 1 is a section view of a printed wiring board in an
embodiment, cut along a line crossing the centers of the cavity areas for
storing representative electronic components. In this figure, a printed
wiring board 10 has a sheet-shaped metal core 11 made of conductive,
rigid metal or typically copper, and multi-layer wiring layers
respectively formed on the top and bottom sides (both principle sides) of
this metal core 11. Note that the term "top and bottom" refers to the
vertical direction of the figure as viewed from the front side of the
figure. The multi-layer wiring layer positioned on the bottom side of the
metal core 11 is hereinafter referred to as the bottom layer 16, while
the multi-layer wiring layer positioned on the top side is referred to as
the top layer 17.

[0059] The metal core 11 has, at a key location or locations, one or more
through-openings connecting the top and bottom sides of the metal core 11
for the purpose of installing electronic components, or in other words, a
rectangular through-opening or openings connecting the front and back
sides of the metal core 11. In the example shown in this figure, two
openings are formed, or specifically an opening 12 positioned on the left
side of the figure and an opening 13 positioned on the right side. The
right opening 13 is used as a cavity area 14a for storing a short
electronic component 14 whose height is equal to or less than the
thickness of the metal core 11, while the left opening 12 is used as a
cavity area 15a for storing a tall electronic component 15 whose height
is far greater than the thickness of the metal core 11. Note that the
inner side face of the left opening 12 is covered with an insulator 21a
having a specified thickness (the same as a first insulation layer 21
explained later), and therefore the practical opening part of this
opening 12, or specifically the part practically used as the cavity area
15a, excludes the thickness of this insulator.

[0060] Examples of these short electronic component 14 and tall electronic
component 15 include capacitors, resistors, integrated circuits and
transistors, among others, and an inductor is a representative example of
the tall electronic component 15. An inductor is formed by storing a
bobbin, around which coil has been wound, in a vertically long, cubic
case. Because of this structure, size reduction is more difficult with
inductors than capacitors, resistors, and other electronic components,
and consequently inductors tend to be taller.

[0061] Holes 39a, 40a are open at two locations, one each on the left and
right, of the insulator 21a formed on the inner side face of the cavity
area 15a. These holes 39a, 40a have bridges 39, 40 (both bridges 39, 40
have been etched) left on the walls.

[0062] In this embodiment, the bottom layer 16 has a layered structure
constituted by a first insulation layer 18, a second insulation layer 19
and a gold plating 20 in order from the metal core 11, and similarly the
top layer 17 has a layered structure constituted by a first insulation
layer 21, a second insulation layer 22, and a gold plating 23 in order
from the metal core 11. In the example shown in the figure, the bottom
layer 16 and top layer 17 both have a two-insulation-layer structure, but
they are not at all limited to such structure. They can have a
multi-layer structure having more than two layers.

[0063] The top layer 17 further has conductive parts 24a, 25a, 26a, 27a
formed at connection locations for constituting an electronic circuit on
the surface of the first insulation layer 21 (these connection locations
and other terms indicating the same are hereinafter simply referred to as
"key locations"), wiring patterns 24b, 25b, 26b, 27b formed on the
surface of the second insulation layer 21, conductive parts 24c, 25c,
26c, 27bc formed at key locations on the surface of the second insulation
layer 22, and wiring patterns 24d, 25d, 26d, 27d formed at key locations
on the surface of the second insulation layer 22, and the foregoing are
inter-connected electrically to constitute a first electrode 24 through
fourth electrode 27.

[0064] On the other hand, the bottom layer 16 further has conductive parts
28a, 28b, 29a, 29b formed at key locations on the first insulation layer
18, and wiring patterns 28c, 29c formed at key locations on the second
insulation layer 19, and the foregoing are connected electrically to
constitute a fifth electrode 28 and sixth electrode 29. Here, the second
insulation layer 19 of the bottom layer 16 not only has the
aforementioned wiring patterns 28c, 29c, but also a reinforcement pattern
30 made of the same material as these wiring patterns 28c, 29c.

[0065] FIG. 2 is a plan view of the reinforcement pattern 30 corresponding
to the plan over the area of view A-A in FIG. 1 as viewed from the bottom
side. In (a), the hatched reinforcement pattern 30 is on the same side as
the wiring patterns 28c, 29c formed at the locations on the first
insulation layer 18, and it also surrounds these wiring patterns 28c, 29c
while also separating and electrically insulating itself from these
wiring patterns 28c, 29c. In the figure, a small gap a is provided.

[0066] The reinforcement pattern 30 has a rectangular outer shape
(horizontally long rectangle in the example shown in the figure), and its
vertical dimension b and horizontal dimension c are set larger than the
vertical dimension d and horizontal dimension e of the opening 12 formed
in the metal core 11. In other words, the magnitude correlations "b>d"
and "c>e" hold true.

[0067] Now, in the figure, the difference between b and d on one side is
given by x and the difference between c and e on one side is given by y.
X and y indicate distances between the broken line representing the edge
of the opening 12 and outline of the reinforcement pattern 30, or
specifically the area where the reinforcement pattern 30 and metal core
11 overlap each other. In other words, the border areas of x and y in
width are facing the metal core 11 in the periphery of the hatched
reinforcement pattern 30 shown in (b).

[0068] Next, the manufacturing process is explained.

[0069] FIGS. 3 to 7 are section views showing the manufacturing process of
the printed wiring board in the embodiment. In these sections, the
section of the opening 12 corresponds to the crank-shaped section passing
the centers of bridges 39, 40 in FIG. 8.

[0070] (1) First step - - - FIG. 3(a)

[0071] First, locations where electronic components are to be stored are
removed from the metal core 11 to form openings 12, 13. Note that with
the opening 12 on the left side of the figure, the inner side of the
opening 12 is not completely removed, but an island-like shape 38
connecting to the metal core 11 via bridges 39, 40 is left, as shown in
FIG. 8. A trench-shaped opening 38a is provided between the island-like
shape 38 and metal core 11 around it, and the bridges 39, 40 are passed
over this opening 38a.

[0072] The following is a brief explanation of how to form the island-like
shape 38 and bridges 39, 40. First, form the top side shown in FIG. 3(a)
by, for example, printing an etching mask or attaching a film over the
entire surface, except for the part where a trench-shaped opening 18a is
to be formed, followed by exposure and development. Next, for the bottom
side shown in FIG. 3(a), form an etching mask over the entire surface
except for the parts where the trench-shaped opening 18a and bridges 39,
40 are to be formed. When etching is done this way, the trench-shaped
opening 18a is formed as a through-hole, while at the same time the
bridges 39, 40 are left only on the top side because on the bottom side
the holes are formed as a result of etching, and consequently an
island-like shape 38 connected via the bridges 39, 40 to the area around
the opening 18a is formed, as shown in the section view in FIG. 3(a).

[0073] One role of the aforementioned opening 18a is to allow an
insulation material to be filled in the opening 18a when the insulation
layer 21 is press-formed on the top layer 16, so that an insulation wall
will be formed when an electronic component is stored in a subsequent
step. Another role is to make it easier to remove the island-like shape
38 in a subsequent step.

[0074] The function of the bridges 39, 40 is to maintain the mechanical
strength needed to securely link the island-like shape 38 to the metal
core 11, but this linking function is only temporary. This is because
once the island-like shape 38 is removed in a subsequent step (refer to
FIG. 5(c)), these bridges 39, 40 are no longer necessary. A desired
embodiment where these bridges 39, 40 can display their intended function
is explained below (quantity, size, thickness, and position in vertical
direction).

[0075] First regarding the "quantity," desirably there should be many (at
least three) bridges so that the island-like shape 38 can be securely
linked to the metal core 11 and required strength maintained. If there
are many bridges, however, many holes, such as etched bridge holes (refer
to holes 39a, 40a in FIG. 1) will remain in the inner wall of the cavity
area 15a once the island-like shape 38 is removed, which is not
desirable. If there are not enough bridges (typically this means there is
only one bridge), on the other hand, the island-like shape 38 is linked
to the metal core 11 in a cantilever configuration and sufficient
mechanical strength cannot be achieved. This means that the
aforementioned function may not be achieved, which is undesirable. Still,
such quantity (one bridge) may be adopted as long as it is confirmed,
through sufficient verification, that the aforementioned function can be
achieved. For example, sometimes even one bridge can achieve the
aforementioned function if the bridge is wide. Accordingly, in practical
settings the number should be more than one, but less than three, or
specifically two, and the number of bridges 39, 40 in the embodiment (two
bridges) was determined based on this concept. In other words, this
number (two bridges) only represents the quantity adopted by the best
embodiment (best mode). In principle, any number can be used as long as
the specified mechanical strength can be achieved.

[0076] Next, the "size (length, width)" of bridges 39, 40 is explained.
The smaller the bridges are, the better, because the bridges will be
removed in a subsequent step. Also, any desired "width" can be set for
bridges 39, 40. For example, they can have a width of 0.3 mm or 0.2 mm,
or 0.1 mm. Note that any width smaller than 0.1 mm makes it difficult to
create bridges by etching, so applying such small width at
mass-production level would be difficult.

[0077] Next, the "thickness" of bridges 39, 40 is explained. An
appropriate thickness of bridges 39, 40 is determined in relation to the
thickness of the metal core 11. If the bridges 39, 40 have the same
thickness as the metal core 11, for example, the aforementioned "holes"
(etched bridge holes; refer to holes 39a, 40a in FIG. 1) become larger
accordingly, and large areas of etched bridge 39, 40 surfaces are
exposed, which is not appropriate. Accordingly, it is desirable to keep
the bridges 39, 40 less thick than the metal core 11. Although the
specific value varies depending on the thickness of the metal core 11, if
the metal core 11 is approx. 1 mm thick, for example, then the bridges
39, 40 can have a thickness of approx. 0.5 mm, roughly one half the core
thickness. Note, however, that this value (0.5 mm) is only a reference.
Any thickness can be used as long as it has been reduced as much as
possible and also permits forming and removal in the manufacturing
process.

[0078] Next, the "position (position in a section view)" of bridges 39, 40
is explained. If the bridges 39, 40 are not as thick as the metal core
11, the bridges 39, 40 can be positioned (a) farthest from the bottom of
the cavity 15a, (b) closest to the bottom of the cavity 15a, or (c)
between these two positions. Any of these potions can be selected. Here,
(a) was selected in the embodiment.

[0079] The island-like shape 38 is explained.

[0080] FIG. 8 is a plan view showing the island-likes shape 38. As
illustrated, the island-like shape 38 is linked to the metal core 11 via
the bridges 39, 40 formed on the opposing sides at the left and right of
the figure, and the horizontal dimension f and vertical dimension g of
this island-like shape 38 are roughly the same as the horizontal
dimension and vertical dimension of the opening of the cavity area 15a
for storing the tall electronic component 15. Also, a trench-shaped
opening 38a is provided around the island-like shape 38, and the bridges
39, 40 are formed in a manner dividing this opening 38a into two. Here,
if the vertical width of the opening 38a in the figure is given by h and
lateral width of the opening 38a is given by i, the relationships
"f+2i<c" and "g+2h<b" hold true. Here, c and b are the vertical and
horizontal dimensions of the reinforcement pattern 30 (c is the
horizontal dimension, while b is the vertical dimension). As long as
these relationships (f+2i<c, g+2h<b) are met, the relationships of
the reinforcement pattern 30 and opening 12 in FIG. 2, or "b>d" and
"c>e," are met and an overlap for adjusting strength (hatched part in
FIG. 2(b)) can be provided between the reinforcement pattern 30 and metal
core 11.

[0081] As a method for removing the bridges 39, 40 from the metal core
when the openings 12, 13 are formed, metal etching, cutting, etc. can be
used, and any other methods can also be adopted.

[0082] (2) Second step - - - FIG. 3(b)

[0083] Next, the first insulation layer 18 is formed on the bottom side of
the metal core 11 and the openings 12, 13 are closed on one side with the
insulation layer (bottom layer) to form their bottoms, and consequently
the opening 13 is used as the cavity area 14a for storing the short
electronic component 14. The material for the first insulation layer 18
is not specifically defined. In essence, as long as it has electrical
insulation property, any material such as resin, ceramics or other
material used alone, or glass fiber or nonwoven fabric impregnated with
resin can be used. Also the bottom side of the metal core 11 may be
chemically or physically treated to improve the adhesion between the
first insulation layer 18 and metal core 11.

[0084] (3) Third step - - - FIG. 3(c)

[0085] Next, the short electronic component 14 is mounted in the cavity
area 14a.

[0086] (4) Fourth step - - - FIG. 4(a)

[0087] Next, the first insulation layer 21 is press-formed on the top side
of the metal core 11. The specific material for this first insulation
layer 21 should be any material that has electrical insulation property
and can completely fill any gaps around the openings 12, 13 in the metal
core 11. For example, the first insulation layer 21 may use such
materials as resin and ceramics. In this step, any chemical or physical
treatment can be added to the insulation material with the intent of
improving the adhesion with the metal core 11.

[0088] (5) Fifth step - - - FIG. 4(b)

[0089] Next, holes 41 to 46 are opened in the locations where conductive
members used for electrical circuit connections are to be formed, by
means of laser cutting or drilling, for example, in the first insulation
layers 18, 21 on the top and bottom sides of the metal core 11.

[0090] (6) Sixth step - - - FIG. 4(c)

[0091] Next, these holes 41 to 46 are metal-plated on the inside (or on
inner walls) or filled with conductive paste to make them conductive
parts 24a, 25a, 26a, 27a, 28a, 29a, and then metal films 100, 101 made of
copper, etc., are formed on these conductive parts 24a, 25a, 26a, 27a,
28a, 29a, after which these metal films 100, 101 are patterned to form
wiring patterns 24b, 25b, 26b, 27b, 28c, 29c.

[0092] Here, these conductive parts 24a, 25a, 26a, 27a, 28a, 29a and
wiring patterns 24b, 25b, 26b, 27b, 28c, 29c, together with the
conductive parts 24c, 25c, 26c, 27c and wiring patterns 24d, 25d, 26d,
27d formed in subsequent steps (such as the twelfth and fourteenth
steps), constitute the first electrode 24 through fourth electrode 27
positioned on the top side of the metal core 11 as well as fifth
electrode 28 and sixth electrode 29 positioned on the bottom side of the
metal core 11.

[0093] This embodiment is characterized in that in this sixth step, a
reinforcement pattern 30 is formed on the bottom side of the metal core
11 at the same time when the metal films 100, 101 are patterned to form
wiring patterns 24b, 25b, 26b, 27b, 28c, 29c. In other words, while
traditionally (according to the background art) only wiring patterns 28c,
29c are formed on the bottom side of the metal core 11, this embodiment
is characterized, structurally, in that the wiring patterns 28c, 29c and
reinforcement pattern 30 are formed simultaneously from the metal film
101.

[0094] This enhances the mechanical strength at the bottom of the cavity
areas 14a, 15a for storing components as formed on the metal core 11, to
avoid worst-case scenarios such as the bottoms detaching. In this stage,
however, only the cavity area 14a is formed and the cavity area 15a is
not yet formed. The cavity area 15a is formed in the ninth step (step
shown in FIG. 5(c)) explained later.

[0095] (7) Seventh step - - - FIG. 5(a)

[0096] Next, the second insulation layers 19, 22 are formed on the top and
bottom sides of the metal core 11. The specific material for these second
insulation layers 19, 22 may be resin, ceramics or other material used
alone, or resin, ceramics or other material mixed with glass fiber or
nonwoven fabric and then formed, for example. If necessary, any chemical
or physical treatment can be added with the intent of improving the
adhesion with the first insulation layers 18, 21.

[0097] If the bottom layer 16 and top layer 17 of the metal core 11 are to
have a multi-layer structure constituted by three or more layers, simply
repeat the above insulation-layer forming process and conductive-layer
forming process (second and fourth through seventh steps).

[0098] (8) Eighth step - - - FIG. 5(b)

[0099] Next, laser light is irradiated onto the surface, where an opening
17a is to be formed, of the first insulation layer 21 and second
insulation layer 22 of the top layer 17, to remove the opening area and
thereby form the opening 17a through which the island-like shape 38 of
the metal core 11 becomes exposed.

[0100] (9) Ninth step - - - FIG. 5(c)

[0101] Next, this opening 17a is masked and the remaining surface,
including the bridges 39, 40 linked to the island-like shape 38 of the
metal core 11, is etched to remove the island-like shape 38 and connect
non-through parts, thereby forming, for example, the cavity area 15a for
storing the tall electronic component 15. When this island-like shape 38
is removed, the bridges 39, 40 are also etched around the masked area.
The bridges 39, 40 illustrated have their walls concave like curved
surfaces beyond the masked area, but this concavity is a general
phenomenon associated with etching. In the meantime, holes 39a, 40a are
formed in the insulator 21a on the inner side face of the cavity area
15a. These holes 39a, 40a are formed at the same time when the walls of
bridges 39, 40 are etched beyond the masked area during the etching of
bridges 39, 40.

[0102] It is also possible not to use the etching method, in which case
the opening 17a in the first insulation layer 21 and second insulation
layer 22 of the top layer 17 is formed by means of laser cutting or
drilling, while at the same time the metal core 11 is made free of any
divisions to remove the island-like shape 38, in the eighth step above.

[0103] The cavity area 15a for storing an electronic component, as formed
above, has a unique benefit in that it can prevent unwanted electrical
connection (shorting) between the metal core 11 and any electronic
component mounted in the cavity area 15a, such as the tall electronic
component 15, because the inner wall of the opening 12 is covered with
the insulator 21a (formed simultaneously with the first insulation layer
21).

[0104] (10) Tenth step - - - FIG. 6(a)

[0105] Next, holes 47, 48 are opened in the first insulation layer 18
forming the bottom of the cavity area 15a, by means of laser cutting,
counterboring, etc., to expose the wiring patterns 28c, 29c buried in the
first insulation layer 18 and second insulation layer 19.

[0106] (11) Eleventh step - - - FIG. 6(b)

[0107] Next, holes 49 to 52 are opened also in the second insulation layer
22 of the top layer 17, by means of laser cutting, counterboring, etc.,
to expose the wiring patterns 24b, 25b, 26b, 27b buried in the second
insulation layer 22. This eleventh step and above tenth step may be
swapped.

[0108] (12) Twelfth step - - - FIG. 6(c)

[0109] Next, the holes 49 to 52 formed in the eleventh step are
metal-plated on the inside or on inner walls or filled with conductive
paste to make them conductive parts 24c, 25c, 26c, 27c, while at the same
time a metal film made of copper, etc., is formed on these conductive
parts 24c, 25c, 26c, 27c and this metal film is patterned to form wiring
patterns 24d, 25d, 26d, 27d.

[0110] (13) Thirteenth step - - - FIG. 7(a)

[0111] Next, solder resist is formed on the top layer 17 and gold plating
23 is applied on the surface layer electrodes.

[0112] (14) Fourteenth step - - - FIG. 7(b)

[0113] Next, solder, conductive adhesive, anisotropic conductive adhesive
or other conductive material is coated, by the dispenser method, etc., on
the holes 47, 48 in the first insulation layer 18 forming the bottom of
the cavity area 15a, to provide conductive parts 28b, 29b. These
conductive parts 28b, 29b, together with the conductive parts 28a, 29a
and wiring patterns 28c, 29c formed earlier, constitute the fifth
electrode 28 and sixth electrode 29.

[0114] (15) Fifteenth step - - - FIG. 7(c)

[0115] Next, an electronic component, such as the tall electronic
component 15, is mounted in the cavity area 15a, followed by
post-treatment appropriate for each material, such as heat treatment to
melt solder. In this example, an inductor is used as the electronic
component mounted in the cavity area 15. Note that this is only one
example of an electronic component taller than the depth of the cavity
area 15. Any component can be used as long as it is taller than the metal
core.

[0116] Even when the component is shorter than the metal core, a
reinforcement pattern 30 may still be formed to reinforce mechanical
strength, if any force is applied to the electronic component in the step
to store the electronic component.

[0117] By implementing the above steps (first through fifteenth steps), a
printed wiring board 10 having the structure shown in FIG. 1 can be
manufactured.

[0118] The next (sixteenth) step may be performed after the above
fifteenth step.

[0119] (16) Sixteenth step

[0120] FIG. 9 is a manufacturing process diagram for the printed wiring
board in the embodiment, showing the sixteenth step.

[0121] In this step, the printed wiring board 10 manufactured through the
above first through fifteenth steps is covered with a shield case 49.
This shield case 49 has been formed in the shape of a box with its bottom
side open, using aluminum or other metal material, resin material (such
as plastic) on which conductive film has been formed, or other material
having an electromagnetic shielding effect. Its open end faces 49a, 49b
are electrically connected to the side faces of the metal core 11. This
way, inside and outside of the printed wiring board 10 can be
electromagnetically shielded by the shield case 49, to protect against
EMI (prevent irradiation of electromagnetic waves to the outside and
entry of electromagnetic waves from the outside).

[0122] According to the aforementioned structure and manufacturing
process, this embodiment provides the following effects: [0123] (a)
Existence of the reinforcement pattern 30 at the bottom of the cavity
area 15a for storing the relatively tall electronic component 15 prevents
the bottom of the cavity area 15a from cracking or detaching even when,
for example, the head of the tall electronic component 15 is pressured in
any way (refer to P in FIG. 18). As a result, a printed wiring board 10
offering excellent mechanical strength and high reliability can be
achieved. [0124] (b) The reinforcement pattern 30 is formed by
effectively utilizing the unused parts of wiring patterns 28c, 29c. To be
specific, when wiring patterns 28c, 29c are formed from the metal film
101 in the sixth step (FIG. 4(c)), the reinforcement pattern 30 is formed
simultaneously from the same metal film 101. This way, the wiring
patterns 28c, 29c and reinforcement pattern 30 can be formed
simultaneously in one step (sixth step), which eliminates the need for an
additional step and prevents cost increase. This is also a
resource-saving measure, because the parts that would otherwise be
removed are effectively utilized to form the reinforcement pattern 30.
[0125] (c) Also, an insulator is formed on the inner wall of the cavity
area 15a for storing the tall electronic component 15, which is formed
simultaneously with the first insulation layer 21, and this provides a
unique benefit of preventing unwanted electrical connection (shorting)
between the metal core 11 and electronic component (tall electronic
component 15) mounted in this cavity area 15a. [0126] (d) In addition,
this cavity area 15a for storing the tall electronic component 15 can
also be formed for a very thin multi-layer printed wiring board (such as
one with a thickness of approx. 1 mm), which contributes to development
of thinner electronic devices. This is because the present invention
includes a step to form an island-like shape 38 on the metal core 11, a
step to form wiring layers on both sides of this metal core 11, a step to
open one wiring layer, and a step to utilize this opening to remove the
aforementioned island-like shape 38, thereby allowing the area left after
removing the island-like shape 38 to be used as the cavity area 15a for
storing the tall electronic component 15. The present invention also
includes a step to fill with an insulator 21a the trench-shaped opening
38a formed around the island-like shape 38, which allows the side wall of
the cavity area 15a to be covered with the insulator 21a, thereby
avoiding shorting between the tall electronic component 15 and metal core
11.

[0127] Next, variations of the embodiment are explained.

[0128] FIG. 10 shows a first variation of the reinforcement pattern 30. In
this first variation, the reinforcement pattern 30 has many small holes
31 formed in an array at an equal pitch, or at irregular pitches or in a
random manner. This way, the first insulation layer 18 and second
insulation layer 19 come in contact with each other through the small
holes 31, which increases the adhesion between the two insulation layers
18, 19. Note that the shape of small holes 31 is not limited to the one
illustrated (circle). They can have any other shape, such as oval,
rectangle or diamond. The area ratio of small holes 31 should preferably
be 20 to 30% of the total area. If the formed holes occupy more area, the
fundamental purpose of the reinforcement pattern 30 will be lost and
mechanical strength will drop.

[0129] FIG. 11 shows a second variation of the reinforcement pattern 30.
In this second variation, the reinforcement pattern 30 is comprised of a
first reinforcement pattern 30a and a second reinforcement pattern 30b at
the left and right in the figure. In other words, the reinforcement
pattern 30 is divided into two and a space 30c is provided in between. In
this case, cross wirings may be formed in this space 30c.

[0130] FIG. 12 shows a third variation (a) and fourth variation (b) of the
reinforcement pattern 30. In the third variation (a), the reinforcement
pattern 30 is divided into, and comprised of, four triangle reinforcement
patterns (first reinforcement pattern 32 through fourth reinforcement
pattern 35). The first reinforcement pattern 32 is positioned at the top
left-hand corner of the opening 12, second reinforcement pattern 33 is
positioned at the top right-hand corner of the opening 12, third
reinforcement pattern 34 is positioned at the bottom right-hand corner of
the opening 12, and fourth reinforcement pattern 35 is positioned at the
bottom left-hand corner of the opening 12. As with the second variation,
cross wirings can be formed.

[0131] In the fourth variation (b), the reinforcement pattern 30 is
comprised of one reinforcement pattern of vertically long rectangular
shape 36 covering the top side and bottom side of the opening 12.

[0132] FIG. 13 shows a fifth variation of the reinforcement pattern 30,
where (a) is a plan view of the variation and (b) is a B-B section view
of (a). This reinforcement pattern 30 is such that many via conductors 37
are formed in an array along the four sides of the pattern, and this
array part is where the reinforcement pattern overlaps the metal core
(refer to the hatched area in FIG. 2(b)). These via conductors 37 have
two functions. Their first function is to electrically connect the
reinforcement pattern 30 and metal core 11 through the via conductors 37.
In this fifth variation, therefore, the reinforcement pattern 30 and
metal core 11 can have the same electrical potential and, because the
metal core 11 is generally used at a ground potential, the reinforcement
pattern 30 can also be given a ground potential to shield the bottom of
the opening 12. Their second function is to firmly connect the
reinforcement pattern 30 and metal core 11 through the via conductors 37.
To be specific, the via conductors 37 can be made with metal material to
connect the reinforcement pattern 30 and metal core 11 to create an
integral structure constituted by three members, i.e., via conductors 37,
reinforcement pattern 30 and metal core 11, thereby causing the metal
core 11 to securely support the reinforcement pattern 30 and further
strengthen the bottom of the cavity area 15a. If this second function is
given greater focus, these via conductors 37 can be called "reinforcement
vias." These via conductors 37 can be formed, after the aforementioned
fifth step, together with other vias in the same layer. Needless to say,
the size, quantity and layout of via conductors 37 can be adjusted freely
according to the shape of the reinforcement pattern 30 and other design
items.

[0133] If the reinforcement pattern 30 has a shielding effect, the
following additional benefit is also achieved. Assume that the tall
electronic component 15 stored in the cavity area 15a is an inductor.
Leakage flux from this inductor tends to flow toward lower impedance, and
therefore it flows into the reinforcement pattern 30 having a ground
potential. Since generally leakage flux can lower the L value of the
inductor, ideally it should be zero. However, the L value can be
fine-tuned by utilizing the fact that leakage flux flows into the
reinforcement pattern 30. By changing the shape of reinforcement pattern
30 in different ways, for example, as shown in FIGS. 10, 11 and 12, the
amount of leakage flux flowing into the reinforcement pattern 30 changes
according to each variation, and this has the effect of fine-tuning the L
value.

[0134] FIG. 14 shows a sixth variation of the reinforcement pattern 30. In
this sixth variation, corners 102 to 109 of the wiring patters 28c, 29c
are rounded, and corners 110 to 117 of the reinforcement pattern 30
corresponding to these corners 102 to 109 are also rounded. Here,
"rounding" refers to changing sharp edges into a smooth line. By these
"rounding" operations, high-frequency characteristics can be improved.
This is because a high-frequency electric field tends to concentrate on
sharp edges to cause high-frequency characteristics to change. Once sharp
edges are gone, electric field concentration can be suppressed to
stabilize high-frequency characteristics.

[0135] Note that in the above embodiment, the wiring pattern 30 is formed
by utilizing the unused parts of the wiring patterns 28c, 29c to
reinforce the bottom of the cavity part 15a, but it is also possible to
form the first insulation layer 18, second insulation layer 19, or both,
of the bottom layer 16 with resin containing glass cloth or nonwoven
fabric, in addition to providing the above reinforcement pattern 30. Such
resin containing glass cloth or nonwoven fabric also has a substantial
reinforcement effect and, combined with the above reinforcement pattern
30, it can demonstrate a greater reinforcement effect.

[Second Embodiment]

[0136] Next, the second embodiment is explained. The difference from the
above first embodiment lies in how the cavity area 15a for storing the
tall electronic component 15 is created. In the above first embodiment, a
part of the metal core 11 is left as an island-like shape 38, which is
subsequently removed to form this cavity area 15a. This second embodiment
is different in that the cavity area 15a is created without forming an
island-like shape.

[0137] The manufacturing process of the second embodiment is explained.
Note that in the following explanation, components also found in the
aforementioned first embodiment are denoted by the same symbols.
Accordingly, refer to the aforementioned first embodiment for symbols
that are not explained.

[0138] (1) First step - - - FIG. 15(a)

[0139] Basically this step corresponds to the first step (FIG. 3(a))
through seventh step (FIG. 5(a)) in the first embodiment. The difference
is that only one opening 13 is formed in the metal core 11. In other
words, the opening 12 is not formed in the second embodiment.
Accordingly, the island-like shape 38, bridges 39, 40 and opening 38a are
not formed either.

[0140] (2) Second step - - - FIG. 15(b)

[0141] Next, laser cutting, etc., is used to remove the location where an
electronic component is to be stored, on the first insulation layer 21
and second insulation layer 22 of the top layer 17, to form an opening
17a through which the top surface of the metal core 11 is exposed.

[0142] (3) Third step - - - FIG. 15(c)

[0143] Next, this opening 17a is masked and the metal core 11 is etched to
form a cavity area 15a for storing a tall electronic component 15. At
this time, the inner wall of the cavity area 15a is not a steep vertical
wall like those indicated by broken lines 100, 101. This is because the
rate at which etching progresses varies due to the thickness of the metal
core 11. In reality, the inner wall draws a curve with its skirts at the
bottom of the cavity area 15a. In other words, the area of opening of the
cavity area 15a is the largest near the top close to the opening 17a and
smallest at the bottom side.

[0144] (4) Fourth step - - - FIG. 16(a)

[0145] Next, unnecessary parts around the opening 17a are removed by laser
cutting. If the laser beam reaches the surface of the bottom layer 16
(first insulation layer 18) at this time, the surface of this first
insulation layer 18 may be damaged. In this second embodiment, however,
the cross-section shape of the cavity area 15a expands at the bottom like
skirts, as explained above, and therefore the laser beam can be received
by these "skirts" to avoid damage to the surface of the first insulation
layer 18.

[0146] (5) Fifth step - - - FIG. 16(b)

[0147] Next, holes 47, 48 are opened by means of laser cutting,
counterboring, etc., at the bottom of the cavity area 15a (i.e., first
insulation layer 18) to expose the wiring patterns 28c, 29c buried in the
first insulation layer 18, while at the same time holes 49 to 52 are
opened by means of laser cutting, counterboring, etc., in the second
insulation layer 22 of the top layer 17 to expose the wiring patterns
24b, 25b, 26b, 27b buried in the second insulation layer 22.

[0148] (6) Sixth step - - - FIG. 16(c)

[0149] Next, the holes 49 to 52 formed in the fifth step are metal-plated
on the inside (or on inner walls) or filled with conductive paste to form
wiring patterns 24d, 25d, 26d, 27d.

[0150] (7) Seventh step - - - FIG. 17(a)

[0151] Next, solder resist is formed on the top layer 17 and gold plating
23 is applied on the surface layer electrodes.

[0152] (8) Eighth step - - - FIG. 17(b)

[0153] Next, solder, conductive adhesive, anisotropic conductive adhesive
or other conductive material is coated on the holes 47, 48 at the bottom
of the cavity area 15a to constitute the fifth electrode 28 and sixth
electrode 29.

[0154] (9) Ninth step - - - FIG. 17(c)

[0155] Next, an electronic component (tall electronic component 15) is
mounted in the cavity area 15a, followed by post-treatment appropriate
for each material, such as heat treatment to melt solder. In this
example, an inductor is used as the electronic component mounted in the
cavity area 15. Note that this is only one example of an electronic
component taller than the depth of the cavity area 15. Any component can
be used as long as it is sufficiently tall.

[0156] By implementing the above steps (first through ninth steps), a
printed wiring board having the cavity area 15a for storing the tall
electronic component 15 can also be manufactured.

[0157] According to this second embodiment comprised of the above steps,
there is no need to create the island-like shape 38 in the aforementioned
first embodiment when forming the cavity area 15a for storing the tall
electronic component 15. This also makes the bridges 39, 40 for linking
this island-like shape 38 to the metal core 11 unnecessary, and
consequently no bridge 39, 40 holes (refer to symbols 39a, 40a in FIG. 1)
are left on the inner wall of the cavity area 15a after etching.

[0158] Also in this second embodiment, the bottom of the cavity area 15a
becomes narrow when the cavity area 15a is formed by etching the metal
core 11. This (narrower bottom) may be considered an etching problem, but
in reality it is an advantage. In the fourth step where laser cutting is
used to remove the projected areas at the top of the cavity area 15a, or
unnecessary parts around the opening 17a, the laser beam may damage the
surface of the first insulation layer 18 if it reaches the bottom of the
cavity area 15a (i.e., first insulation layer 18).

[0159] However, the cross-section shape of the cavity area 15a expands at
the bottom like skirts, and therefore the laser beam can be received by
these "skirts" to avoid damage to the surface of the first insulation
layer 18.